Field of the Invention
Thermal energy storage systems are a means for storing energy during certain periods to provide a thermal energy source during other periods.
The specific heat "C.sub.p " and/or the latent heat "L" of a material is the primary factor considered in selecting heat storage materials. Specific heat is the amount of heat required to heat a given amount of the storage material through one degree. Thus, if the storage material is heated through a temperature range of .DELTA.T.degree. F., then the number of BTU's stored is C.sub.p .DELTA.T. On the other hand, latent heat is heat stored at a given temperature by a phase change. Thus, when ice at 0.degree. C. is melted to water at 0.degree. C., 80 calories per gram is absorbed. Likewise, when water at 0.degree. C. freezes to ice at 0.degree. C., the 80 calories of heat are given up to the surroundings.
The use of latent heat storage materials is gaining favor in many heat storage systems. Factors considered in their selection are: the melting point, low cost, high heat of fusion per unit weight, availability in large quantities, and simplicity of preparation. In addition, they may be non-toxic, non-flammable, non-combustible and non-corrosive. The lowest cost materials for use are large volume chemicals based on compounds of sodium, potassium, magnesium, aluminum, and iron. Preferably, the materials are in the form of salt-hydrates and their eutectics. The type of low cost compounds are chlorides, nitrates, sulfates, phosphates, carbonates and their hydrates while additives or modifiers may include borates, hydroxides and silicates.
The primary difficulty encountered in utilizing some latent heat of fusion materials, such as sodium sulfate decahydrate, is that after several cycles of melt-freeze phase transformations, some anhydrous sodium sulfate settles out. When the mixture is cooled below the freezing point again, heavy crystals of sodium sulfate on or near the bottom of the container cannot completely dissolve to crystallize as a salt-hydrate. Sodium sulfate decahydrate, that has been recycled several times, solidifies in three distinct layers; these are (a) a bottom layer of white anhydrous sodium sulfate crystals, (b) a larger intermediate layer of translucent sodium sulfate decahydrate crystals, and (c) on top, a layer of liquid saturated solution of sodium sulfate.
Various thickening agents have been included in heat storage mixtures as additives with the aim of producing a suspension in which the anhydrous salt does not settle out even after successive heating/cooling cycles, but are ineffective.